It is important to not confuse the song structure, 12-bar blues, with the genre of music known as the blues. The 12-bar blues song form is used in many genres of music including country, jazz, rock, folk, and, yes, blues.
It is a simple song form that is based on a 3-chord progression. These three chords, relative to the key of the song, are known as:
The capital Roman numerals represent major chords. So in the key of C Major, the chords would be C (the one), F (the four), and G (the five). To try to make this more clear, think of D as “the two” and E as “the three.” There will be more lessons in chord progressions, but for now, let’s just play with the I, IV, V of the key of C Major: C, F, and G.
…and then repeat, returning back to the C (the I chord)
The numbers in the top row represent the bars of music, let’s say 4 beats each in a 4/4 rhythm (so that the 12 bars altogether have 48 beats), and the the letters in the bottom row represent the chords to play.
Notice the G7 has the 7 added to it. This is what is called a “dominant 7.” It’s “dominant” because when you hear it in this context, your ear really, really, really wants to hear the C chord after it.
There are dozens of variations of this 12-bar blues form. Some of these variations might only start off with two measures of the I chord before jumping to the IV chord. This is known as a “quick two.” In jazz, some of these I, IV, V chords are replaced with similar sounding chords but which might add a colorful note or two to the chord. So instead of playing
IV – V7 – I (i.e. F – G7 – C)
a jazz musician might play
ii7 – V7 – I (i.e. D minor7 – G7 – C).
You have probably heard the 12-bar blues song form many, many times during the course of your whole life. It is easy to follow and is a very simple and powerful way to get your message across, thus it’s popularity.